The present invention concerns an actuator arrangement which allows a defined force to be applied to a body and which can be employed in particular in connection with an optical imaging apparatus. The invention can be employed in the field of microlithography, which is used in connection with the manufacture of microelectronic circuits. The invention further extends to an optical imaging method which can be performed, among other processes, with the optical imaging apparatus according to the invention.
Particularly in the realm of microlithography it is imperative, among other requirements besides using components of the highest possible precision, that the position and geometry of the components of the imaging apparatus, i.e. for example the optical elements such as lenses, mirrors or reticles be kept as constant as possible in order to achieve a high image quality. The high requirements for accuracy in the microscopic range at an order of magnitude of a few nanometers and below are in large part a consequence of the constant need to increase the resolution of the optical systems that are used in the manufacture of microelectronic circuits, in order to further advance the miniaturization of the microelectronic components being manufactured.
As a means to achieve an increased resolution, one can either use light of a shorter wavelength, as is the case in systems operating in the extreme UV range (EUV) at operating wavelengths in the range of 13 nm, or one can increase the numerical aperture of the projection system. One possibility to significantly increase the numerical aperture above a value of 1.0 is realized with so-called immersion systems, where the space between the last optical element of the projection system and the substrate that is to be exposed is occupied by an immersion medium whose refractive index is larger than 1.0. A further increase of the numerical aperture is possible with optical elements of a particularly high refractive index.
With a shorter operating wavelength as well as with a higher numerical aperture, not only will the optical elements that are being used have to meet more stringent requirements on positioning accuracy and on the ability to hold their dimensions over the entire operating life, but there will of course also be increased requirements to minimize the imaging errors of the entire optical arrangement.
A known concept to minimize imaging errors is to subject the optical elements involved to an active deformation in order to change their optical characteristics in such a way as to counteract one or more specific imaging errors of the optical system (even to the extent of completely correcting the imaging error). In order to achieve the desired deformation of the optical element, suitable forces are applied to the optical element through a diversity of actuators.
In many cases, so-called N-wave deformations (where N is an integer larger than 1) are generated in order to effect the correction of imaging errors. Normally, this involves subjecting the optical element to actuator forces (normally parallel to the optical axis of the optical system) which are applied at N locations distributed (in most cases evenly) on the circumference of the optical element. In between the points of application of the actuator forces, the optical element is seated against support elements or further actuators (which are normally set along the circumferential direction halfway between every two neighboring points of application of the actuator forces). The result is, accordingly, a deformation with an undulating shape in the circumferential direction of the optical element. An arrangement of the kind has been described for example in DE 198 27 603 A1 (Holderer et al.), whose entire disclosure is incorporated herein by reference.
This wave-shaped deformation can be used to compensate for imaging errors of the kinds which are caused for example when optical elements of optical systems are heating up. It is normally necessary to superimpose deformations of different order N on each other in order to achieve a desired corrective effect. With an arrangement designed for a certain maximum order N, it is also possible to produce deformations of a lower order. For example, with an arrangement for a 4-wave deformation, it is also possible to produce a 2-wave deformation.
This concept is often implemented with fluidic actuators, which allow a desired actuator force to be generated by setting a corresponding pressure in an actuator chamber. Such fluidic actuators have the advantage that there is an exactly defined relationship between the pressure in the actuator chamber and the actuator force generated by the actuator, so that the actuator force can be regulated simply by regulating the pressure in the actuator chamber.
This arrangement poses the problem that, depending on the geometry of the optical element, a significantly smaller actuator force may be required for a lower-order deformation than for a higher-order deformation. Accordingly, if this is the case, a lower-order deformation will be significantly more sensitive than a higher-order deformation in regard to errors that may occur in setting the actuator force.
Thus, it is possible for example in lenses that are thick at the border, that a 4-wave deformation will require a 20 times larger actuator force than a 2-wave deformation of the same amplitude. This represents a disadvantage in that the pressure regulation has to be designed for the maximum pressure to be generated in the actuator chamber and consequently, if the relative accuracy of the pressure regulation is assumed to be approximately constant, the absolute accuracy of the setting for the smaller actuator forces for the 2-wave deformation is reduced by a corresponding factor.
Finally, besides the aforementioned actuator arrangements, an actuator arrangement for deforming a lens is described in DE 198 27 603 A1 among other subjects, wherein two identical actuators, arranged in diametrically opposite locations (relative to the optical axis of the lens) and acting parallel to the plane of the lens, are introducing bending moments into the lens by way of the lens mount. This arrangement, too, suffers from the aforementioned drawbacks.